properties of hardened concrete - chulapioneer.netserv.chula.ac.th/~pwithit/ce231 6.pdf · ·...

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Properties of Hardened ConcreteProperties of Hardened Concrete

Withit PANSUKDepartment of Civil Engineering

Faculty of EngineeringChulalongkorn University

CE 231 Construction MaterialsJuly 19th, 2011

Outline

• Introduction• Practical Criteria of Strength• Factors in Strength of Concrete g• Development of Strength• Tensile and Compressive Strengths

2CE 231 Construction Materials

Outline

• Fatigue Strength• Abrasion Resistance• Bond to Reinforcement• Elasticity• Creep and Relaxation• Permeability


Introduction

CE 231 Construction Materials 4

Hardened concrete (After Final set)

Introduction

• Properties of hardened cement paste, depend on the physical structure of hydration more than chemical compositionhydration more than chemical composition in cement paste

• In many practical cases the durability, permeability and volume stability of concrete are the most important properties


Introduction

• Flaws, microcracking, discontinuities and pores are significance in concrete durability but they are very difficult to quantify in a useful manner

• Thus, ‘Strength of concrete’ is considered to be the most valuable property of hardened concrete


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Introduction

• Strength of Concrete depends on– Strength of cement paste– Strength of aggregate– Strength of aggregate– Interface between cement paste and

aggregates


Introduction

• In many practical cases, the aggregates always have the higher strength than cement pastecement paste

• For the factor which effect on the strength of concrete, we will consider only the strength of cement paste and the Interfacebetween cement paste and aggregates


Practical Criteria of Strength

• The most important practical factor is the W/C, but the underlying parameter is the number and size of pores in the hardenednumber and size of pores in the hardened cement paste


Practical Criteria of Strength

Practical Criteria of Strength :– Porosity– Total void in concrete– Pore size distribution– Microcracking and Stress-Strain

Relation


Porosity

Pores in hydrated cement paste• The hydrated cement paste contains

several types of pores which have an yp pimportant influence on its properties– Gel pores (Interlayer space in C-S-H)– Capillary pores– Air voids


Porosity

Gel Pores : • The gel pores are very small (about 2 nm

i di t ) d th l f l t iin diameter) and the volume of gel water is about 28% of the cement gel

• The pore size is too small to have an adverse effect on the strength and permeability of the hydrated cement paste


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Porosity

• ‘Gel Water’ can be held by hydrogen bonding, and its removal under certain conditions may contribute to drying y y gshrinkage and creep

• In addition to gel water, there exists ‘Combined Water’, which is combined chemically or physically with the product of hydration, and is thus held very firmly


Porosity

• The quantity of combined water can be determined as the non-evaporable water content and in fully hydrated cementcontent, and in fully hydrated cement represents about 23% of the mass of dry cement


Porosity


Diagrammatic representation of the volumetric proportions:(a) before hydration (b) during hydration

Porosity

Capillary pores :• Capillary pores represent the space not

fill d b th lid t f thfilled by the solid components of the hydrated cement paste

• Capillary pores are much larger than gel pores (diameter about 1mm)


Porosity

• For fully hydrated cement with no excess water above the required for hydration, capillary pores is about 18.5% of the p y p %original volume of dry cement

• Capillary pores can be empty or full of water, depending on the amount of water in the mix


Porosity

• The mix contained more water than necessary for full hydration, the capillary pores will excess 18.5% and these are full of water

• The W/C is the main influencing factor on porosity

• The porosity will decrease if cement paste increase the degree of hydration


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Porosity

Influence of W/C and


degree on hydration on capillary and total

porosities of cement paste

Porosity

• There is a corresponding relation between porosity and strength, and this is independent of whether the capillary poresindependent of whether the capillary pores are full of water or empty


Porosity

Relation between i


compressive strength and logarithm of porosity of

cement paste

Porosity

Air Voids : • Air voids are generally spherical• A small amount of air usually gets trapped

in the cement paste during concrete mixing

• Admixture may be added to concrete to entrain purposely tiny air voids


Porosity

• 2 Types of Air Voids– Entrapped air voids :

May be as large as 3 mm. – Entrained air voids :

Usually range from 50 – 200 μm


Porosity

• Both entrapped and entrained air voids in hydrated cement paste are much bigger than capillary voids and are capable of yadversely affecting the strength

• The total amount of voids in concrete can be calculated by the same concept as cement paste associated with the mix proportion


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Porosity


Total Voids in Concrete

Volumetric proportions of concrete of mix proportions 1:2:4 by


proportions 1:2:4 by mass (W/C = 0.55 & entrapped air =2.3 %)(a)before hydration(b)when the degree of

hydration is h=0.7

Pore Size Distribution

• Capillary pore are much larger than gel pores, and there is a whole range of pore sizes throughout the hardened cementsizes throughout the hardened cement paste

• When cement is partly hydrated, the paste contains an interconnected system of capillary pores



• The effect of this is a lower strength and, through increased permeability, a higher vulnerability to freezing and thawing and tovulnerability to freezing and thawing and to chemical attack

• This vulnerability depends also on W/C



• These problems are avoided if the degree of hydration is sufficiently high for the capillary pore system to become p y p ysegmented through partial blocking by newly developed cement gel

• If so, the capillary pores are interconnected only by the much smaller gel pores, which are impermeable.




(a) High permeability - capillary pores interconnected by large passages

(b) Low permeability - capillary pores segmented and only partly connected.

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in hydrated cement paste(Vary in W/C)


Pore Size Distribution in


hydrated cement paste (vary in age)

Microcracking and Stress-Strain Relation

• Such microcracking occurs as a result of differential volume changes between the cement paste and the aggregatecement paste and the aggregate

• These cracks remains stable and do not grow under stress up to 30% of the ultimate strength of concrete


• At stress higher that 30% of ultimate strength, the microcracks begin to increase in length width and number


increase in length, width and number• In consequence the strain increases at a

faster rate than stress = ‘slow propagation of microcracking’



• If the lateral strain is observed, it was found that, the ratio of lateral strain to axial strain (Poisson’s ratio) is constant for stresses below approximately 30% of the ultimatebelow approximately 30% of the ultimate strength

• Beyond this point, Poisson’s ratio increases slowly, and at 70-90% it increases rapidly due to the formation of mainly vertical unstable cracks



• At this stage, the specimen is no longer a continuous body as shown by volumetric strain curvestrain curve

• There is a change from slow contraction in volume to a rapid increase in volume


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• At 70-90% of ultimate strength, cracks open through the matrix and thus bridge the bond crack so that a continuous crack pattern is f d (f t ti f k )formed (fast propagation of cracks)

• If the load is sustained, failure will probably occur with the passage of time

• If the load is increased, rapid failure will take place at the nominal ultimate strength





Stress-strain relations for


cement paste, aggregate, and

concrete


Stress-strain relation for


concretes tested at a

constant rate of strain

Factors in Strength of Concrete

• Although porosity is a primary factor influencing strength, it is a property difficult to measure or even to calculateto measure or even to calculate

• Similarly, the influence of aggregate on microcracking is not easily quantified



• For these reasons, the main factors on strength are taken in practice as – Water – Cement RatioWater Cement Ratio– Degree of compaction– Age and temperature


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• However, there are also other factors such as – Aggregate/cement ratio,– Quality of aggregate– The maximum size of aggregate

• These factors are considered secondary factors when usual aggregates up to a maximum size of 40 mm are used



Relation between t th d


strength and water/cement ratio of concrete


Influence of age on compressive strength of


OPC concrete at different W/C


Influence of the t / t


aggregate/cement ratio on strength of concrete


Effect of max.


aggregate size

Development of Strength

• To obtain good quality concrete, ‘Curing’ during the early stage of hardening must be done

• Curing = the procedures used for promoting the hydration of cement

• The curing procedures being control of the temperature and of the moisture movement from and into concrete


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• The latter affects not only strength but also durability

• The object of curing is to keep concreteThe object of curing is to keep concrete saturated, until the originally water-filled space in the fresh cement paste has been occupied to the desired extent by the products of hydration



• The necessity for curing arise from the fact that hydration of cement can take place only in water filled capillaries (loss of wateronly in water-filled capillaries (loss of water by evaporation from concrete must be prevented)



• If, however, curing proceed until the capillaries in the hydrated cement have become segmented then concrete willbecome segmented, then concrete will impermeable and this is vital for good durability



Influence of curing conditions on


conditions on strength of test

cylinders


Influence of moist curing on


moist curing on the strength of concrete with a W/C of 0.50


• The period of curing can not be prescribed in a simple way

• If the temperature is above 10oC, ACI lays p , ydown a minimum of 3 days for Portland cement type III, 7 days for type I, and 14 days for type IV

• The temperature also affects the length of the required period of curing


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Development of StrengthMinimum period of protection required for different

cements and curing conditions, (by BS 8110: Part 1: 1985)


Development of StrengthMinimum period of protection required for different

cements and curing conditions, (by BS 8110: Part 1: 1985)


Influence of Temperature

• The higher the temperature of the concrete at placement the greater the initial rate of strength development but theinitial rate of strength development, but the lower long-term strength

• This is why important to reduce the temperature of fresh concrete when concreting in hot climate



• The explanation is that a rapid initial hydration causes a non-uniform distribution of the cement gel with a poorerdistribution of the cement gel with a poorer physical structure, which is probably more porous than the structure developed at normal temperatures



• With a high initial temperature, there is insufficient time available for the products of hydration to diffuse away from theof hydration to diffuse away from the cement grains and for a uniform precipitation in the interstitial space



• As a result, a concentration of hydration products is built up in the vicinity of the hydrating cement grains, a process which y g gretards subsequent hydration and the development of longer-term strength

• The influence of the curing temperature on strength indicated a higher initial strength development, but lower 28 days strength


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Relation between compressive


strength and curing time of neat cement paste compacts at

different curing temperatures


Relation between compressive strength

and curing time of neat


and curing time of neatcement paste compacts

at different curing temperatures

(W/C = 0.14; OPC)



Tensile and Compressive Strengths

• The theoretical compressive strength was stated to be 8 times larger than tensile strength

• In fact, The ratio of the two strengths depends on the general level of strength of the concrete

• The ratio of tensile/compressive strengths is lower the higher compressive strength


Tensile and Compressive Strengths


Relation between tensile and compressive strengths of concrete made with normal weight and lightweight aggregates

Fatigue Strength

• Two type of failure in fatigue can take place in concrete– 1st failure occurs under a sustained load (or– 1 , failure occurs under a sustained load (or

slowly increased load). This is known as static fatigue or creep rupture.

– 2nd, type occurs under cyclic or repeated load, and is known simply as fatigue.


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Fatigue Strength

• In both instances, a time-dependent failure occurs only at stress which are greater than a certain threshold value but smaller than the short-term static strength

• At rapid rates of loading, concrete appears more brittle in nature than under lower rates of loading when creep and microcracking increase the strain capacity


Fatigue Strength

• Under low rates of loading, static fatigue occurs when the stress exceeds about 70 to 80 per cent of the short-term strengthto 80 per cent of the short-term strength

• This level represents the onset of rapid development of microcracks


Fatigue Strength• A similar phenomenon takes place under a

sustained load, a certain load is applied fairly quickly and then held constantAb th t l l f 70 80 % f• Above the same stress level of 70 - 80 % of the short-term strength, the sustained load will eventually result in failure

• At the stress level lower that the threshold, failure will not occur and the concrete will continue to creep


Fatigue Strength

Influence of test duration

(or rate of


loading) on strength and

on strain capacity in

compression

Fatigue Strength

Influence of sustained stress on t th d


strength and on strain

capacity of concrete in

compression

Impact Strength

• Impact strength is generally of important in driving concrete piles, foundations for machines exerting impulsive loading andmachines exerting impulsive loading, and when accidental impact is possible

• There is no unique relation between impact strength and compressive strength.


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Impact Strength• For a given type of aggregate, the higher

the compressive strength of the concrete the lower the energy absorbed per blow before cracking but the greater the No ofbefore cracking, but the greater the No. of blows to reach the state of ‘no-rebound’

• The impact strength and total energy absorbed by concrete increase with its static compressive strength and therefore with age at a progressive increasing rate


Impact Strength

• The relation between impact strength and compressive strength depends also upon the type of coarse aggregate but the y gg grelation depends also on the storage condition of the concrete

• The impact strength of water-stored concrete is lower than when concrete is dry


Impact Strength

• For the same compressive strength, impact strength is greater for concrete made with coarse aggregates of greatermade with coarse aggregates of greater angularity and surface roughness

• Thus, impact strength of concrete is more closer related to its flexural strength than compressive strength


Impact Strength

Relation between compressive strength and

No. of blows to ‘no -


rebound’ for concretes made different aggregates and OPC, stored in water

Abrasion Resistance

• Concrete surface can be subjected to various types of abrasive wearSlidi i tt iti I• Sliding or scraping can cause attrition. In the case of hydraulic structures, the action of abrasive solid carried by water leads to erosion of concrete


Abrasion Resistance

Abrasion Resistanceapparatus


apparatus

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Abrasion Resistance

Influence of the W/C of the mix on the abrasion loss


of concrete for different tests

Bond to Reinforcement

• The strength of bond between reinforcement and concrete arises primarily f f i ti d dh ifrom friction and adhesion

• Bond is affected by the properties both of steel and concrete, and by relative movement due to volume change



• In general terms, bond strength is approximately proportional to the compressive strength of concrete up to 20compressive strength of concrete up to 20 MPa

• For higher strength, the increase in bond strength becomes smaller and eventually negligible



Influence of the


strength of concrete on bond determined

by pull-outtest


ASTM C 234Standard Test Method


Standard Test Method for Comparing Concrete on the Basis of the Bond Developed with Reinforcing Steel

Elasticity

• The moisture condition of the specimen is a factor, a wet specimen as a higher modulusmodulus

• The properties of aggregate also influence the modulus of elasticity


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Elasticity

• The influence of the aggregate arises from the value of the modulus of the aggregate and its volumetric proportion, the higher the modulus of aggregates the higher themodulus of aggregates the higher the modulus of concrete

• The relation between the modulus of elasticity of concrete and strength depends also on age, the modulus increases more rapidly than strength


Elasticity

T i l t t i


Typical stress-strain curve for concrete

Elasticity

Typical range of values of 28-day static modulus of elasticity for normal weight concrete, according to BS

8110:Part 2:1985


Elasticity


Creep and Relaxation

• Creep is defined as the increase in strain under a sustained constant stress after taking into account other time-dependenttaking into account other time-dependent deformations



Definition of creep under a constant


under a constant stress s0; E is the secant modulus of elasticity at age t0

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• Creep effects may also be viewed from another standpointIf l d d t i i• If a loaded concrete specimen is restrained so that it is subjected to a constant strain, creep will manifest as a progressive decrease in stress with time

• This phenomenon is termed relaxation



Definition of relaxation for concrete subjected initially to stress s0 and then kept


to stress s0 and then kept at a constant strain; E is the secant modulus of elasticity at age t0

Permeability

• Permeability is the ease with which liquids or gases can travel through concreteThi t i f i t t i l ti t th• This property is of interest in relation to the water-tightness of liquid-retaining structures and to chemical attack


Permeability

• There are no prescribed test methods for permeability, it can be expressed as coefficient of permeability k given bycoefficient of permeability, k, given by Darcy’s equation


Permeability

• dq/dt = the rate of flow of water• A = cross-sectional area of sample• Dh = drop in hydraulic head• L = thickness of the sample


Permeability

• There is no unique relation between air and water permeabilities for any concrete, although they are both mainly dependentalthough they are both mainly dependent on W/C and the age of concrete


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Permeability

Relation between permeability and


p ywater/cement ratio for mature cement

pastes (93 % of cement hydrated)

Permeability

Reduction in permeability of cement paste


cement paste with the

progress of hydration; W/C

ratio = 0.7

Permeability


Concrete Water Permeability Apparatus(Photo from CPAC Concrete testing Lab)

References

• Boonchai Stitmannaithum, "Advance Concrete Technology (Lecture Note)" Chulalongkorn University 2005 : Chapter III Properties of Hardened Concrete

• D. M. ROY and G. R. GOUDA, Porosity - strength relation in y gcementitious materials with very high strengths, J. Amer. Ceramic Soc., 53, No. 10, pp. 549-50 (1973).

• P. T. WANG, S. P. SHAN, and A. E. NAAMAN, Stress-strain curves of normal and lightweight concrete in compression, J. Amer. Concr. Inst., 75, pp. 603-11 (Nov. 1978)


References

• B. G. SINGH, Specific surface of aggregates related to compressive and flexure strength of concrete, J. Amer. Concr. Inst., 54 , pp. 897-907 (April 1958).

• P.KLIEGER, Early high strength concrete for prestressing, y g g p gProc. of World Conference on Prestressed Concrete, pp. A5-1 - A5-14 (San Francisco, July 1957).

• W. H. PRICE, Factors influencing concrete strength, J. Amer. Concr. Inst., 47, pp. 417-32 (Feb. 1951).

• CEMENT AND CONCRETE ASSOCIATION, Research and development on materials, Annual Report, pp. 14-19 (Slough 1976).


References

• CPAC Concrete Academy: The Concrete Product and Aggregate co.,ltd; http://www.cpacacademy.com

• Portland Cement Association (PCA). Cement & Concrete Technology; http://www cement org/Technology; http://www.cement.org/


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CE 231 Construction MaterialsJuly 26th, 2011

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properties of hardened concrete - chulapioneer.netserv.chula.ac.th/~pwithit/ce231 6.pdf · ·...

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